The invention relates to a conversion circuit for converting a differential power signal composed of a first signal supplied on a first common node and a second signal supplied on a second common node, into a differential output current composed of a first and a second current, said circuit having a first output supplying the first current and a second output supplying the second current, said circuit including:
conversion means for converting the first and second signal into the first and second current respectively, said conversion means being connected to the first and second outputs of the conversion circuit,
regulation means for regulating the input impedance of the circuit, which regulation means are connected to the conversion means.
The invention also relates to the domain of mixers and more generally the domain of analog signal processing circuits.
The simplest cell for converting a signal into current uses a transistor subjected to biasing and a resistance, as represented in FIG. 1. The main limitation is that, whatever the type of transistor used, the input impedance rises sharply when the current in the transistor diminishes. This is the case when the absolute value of II is near the value of the bias current IO, which occurs with high power. In this case an a.c. signal will not benefit from constant impedance during conversion, negative values seeing higher impedance than positive values in this time. A costly solution is to increase the bias current. To avoid having to increase the bias current, a differential structure has been proposed by B. Gilbert in U.S. Pat. No. 6,122,497. In this solution, the input signal and the output current are differential. The input current II[1], II[2], respectively, passes through T[1], T[2], respectively during its positive half cycle and through D[1], D[2], respectively during its negative half cycle. The transistors T[1] and T[2] constitute the conversion means while the diodes D[1] and D[2] constitute the regulation means for regulating the input impedance insofar as the current flowing in the diodes during the negative half cycle permits the input impedance of the conversion circuit to be defined during this negative half cycle. Current mirrors enable the current in the diodes to return to the output of the conversion circuit.
The invention takes into consideration a problem of the prior art according to which the input impedance drops on the two sides when the input power increases.
An object of the invention is to reduce the drop of the input impedance when the input power increases. As a result, the invention improves the linearity of the conversion circuit in terms of input impedance.
Indeed a circuit in accordance with the introductory paragraph is characterized according to the invention in that the regulation means include negative feedback means to control a regulating current taken from each component of the power signal as a function of the values of the potentials on the first and second common nodes.
Taking a regulating current from the components of the power signal permits the input impedance to be increased on the side of negative half cycle, whereas it diminishes on the side of the positive half cycle where the regulation only has slight influence on the value of the input impedance. As the functioning of the circuit is differential, the two phenomena compensate each other, leaving the overall impedance constant.
In advantageous embodiments, the conversion and regulation means are implemented with the aid of several transistors arranged in a manner described below with regard to which certain of these are looped to produce a negative feedback. In this case the negative feedback dynamically regulates the biasing of the conversion means so that the operating points are moved. This is preferably achieved by applying a negative feedback potential to a transistor that acts as a current generator. In a preferred embodiment the negative feedback means regulate the regulation current as a function of an average value of the potentials on the first and second common nodes. A common-mode negative feedback is therefore realized. The average value, the sum of the two components of the power signal divided by two, influences the negative feedback means to determine the regulating current.
When transistors are used, the use of a common mode voltage to generate a negative feedback permits the regulating current flowing in the transistors of the regulation means to be controlled. According to the invention, the impedance drops on the side of the positive half cycle and increases on the side of the negative half cycle. The differential input impedance being the sum of the two impedances, the two effects compensate each other and thus make it possible to prevent the drop in the impedance at high power.
Advantageous improvements are proposed in several advantageous embodiments of the invention to optimize the problems of noise. In practice this invention relates to all signal mixers in which a circuit according to the invention is used. Any mixer known as Gilbert type mixer indeed requires a conversion circuit. The mixers are used in signal processing apparatus in which frequency translations are performed: cordless telephones, DECT type telephones, mobile telephones, satellite receivers . . . The invention thus also relates to such signal processing apparatus, the signal being capable of being processed on reception, on transmission or at any time this is necessary.
In the same way the invention relates to apparatus to measure impedance and frequency using a conversion circuit according to the invention within a mixer or otherwise.
The invention will be described below with reference to the attached drawings among which: